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I am building a power supply for a very old laptop. The circuit uses a transformer (220 V primary to 15 V secondary, 33 VA) and that worries me a little. (Please note that I an enthusiast and so far I have been running microprocessors at low voltage and current.)

Since I already had the opportunity to experience how it feels to be a resistor during a power accident at our house last summer, I am considered with my safety a little, not to mention my other fears.

  1. Can 2.2 A of output current cause serious harm to my person or anyone else around?
  2. What happens if I short-circuit the secondary branch by accident?
  3. Is it a proper thing to do to use a 2.5 A fuse given that the laptop is 15 V / 1.8 A? Should I place it:
    • (a) between the transformer and the rest of the circuit, or
    • (b) between the circuit of the power supply and the laptop?
  4. What does it mean when they say the laptop requires a rated power supply? Does it mean the power supply should employ a stabilizer?

My answers, which I would like you to verify:

  1. Hope not.
  2. Sparks, buzz and the transformer dancing, but nothing after all.
  3. It is, but it will burn immediately, because the filtering capacitors will load so eagerly they will burn it. And (a) is right.
  4. It does.
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  • \$\begingroup\$ Interesting reading, this. +1 for that. \$\endgroup\$ – Anindo Ghosh Apr 17 '13 at 18:59
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Can 2.2 A of output current cause serious harm to my person or anyone else around?

Absolutely. From a non-shock perspective, it's enough amps to cause circuit failures to potentially smoulder and burn, or blow low-power devices to bits.

From a shock perspective, it's more than enough to be hazardous, since the mains voltage is high enough to force conduction through your body. On the secondary side of the transformer, not so much, thankfully.

Mains power is not to be toyed with lightly.

What happens if I short-circuit the secondary branch by accident?

The transformer will start drawing lots of current from the mains, causing:

  1. Your appropriately-selected input fuse to open and safely isolate the circuit from the mains, or

  2. Your inappropriate (or absent) fuse to not isolate the circuit, leading to your transformer sitting there and broiling, release some nice-smelling burnt varnish aroma (I call it "power supply incense") into the room. If it has a safety marking on it, it may sit like this indefinitely or fail in a "safe" manner (no dielectric breakdown, no flames or obvious smoke, etc.)

Is it a proper thing to do to use a 2.5 A fuse given that the laptop is 15 V / 1.8 A?

Depends on the load. Fuses not only have to be rated for steady-state current, but should handle things like inrush current (charging those capacitors), line surges and brown-outs without nuisance blows, as well as provide safety during abnormals.

Should I place it (a) between the transformer and the rest of the circuit or (b) between the circuit of the power supply and the laptop?

It must be on the mains side of the transformer. It must prevent any mains voltage from entering the circuit if it blows (which can get complicated if there's a protective earth involved, especially if people fuse the neutral and leave the line unfused). You should also consider a MOV (metal oxide varistor) between the fuse and the transformer across the mains to clamp any surges that come in (it will crowbar and open the fuse).

What does it mean when they say the laptop requires a rated power supply? Does it mean the power supply should employ a stabilizer?

Depends. I would interpret it as meaning regulated, and able to meet the voltage and current requirements of the laptop. I take it you're using the term 'stabilizer' to imply regulation, as that's not a common term in power conversion (at least to my ears).

Other points to ponder:

  • Keep a safe distance between the mains and the output. Creepage and clearance is important for life safety. Don't cross the barrier with any components unless they're safety-agency rated for such an application. Don't cross the barrier with any wires unless they're triple-insulated. Don't connect primary and secondary returns.

  • Consider adding over-voltage, over-current and over-temperature protection. Find a way to shut the thing down or blow the fuse if any of these bad things happen, to keep your downstream stuff from getting damaged by the power supply.

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  • \$\begingroup\$ Madman, that is exactly what I wanted to know! Thank you very much for spending your time. Also thanks to the other guys who helped a lot, too. \$\endgroup\$ – David Apr 17 '13 at 21:13
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Can 2.2 A of output current cause serious harm to my person or anyone else around?

The current rating of the supply is not what determines if it is dangerous. What is dangerous is exposed voltages. If you contact the 240 V mains voltage, you will have an unpleasant experience.

In this circumstance, probably only a few hundred microamps or a few mA of current would be going through your body. But this is plenty enough to disrupt important biological processes like the beating of your heart.

Is it a proper thing to do to use a 2.5 A fuse given that the laptop is 15 V / 1.8 A?

It's a good idea to use a fuse, and your rating is reasonable. But this has nothing to do with the electrocution hazard. The fuse basically prevents your equipment starting a fire if there is a short circuit. It will do nothing at all to protect you if you touch the primary side of the transformer. First because fuses take too long to burn out to protect you from electocution; more importantly because any current shunted to ground through your body from the primary wouldn't be going through a fuse on the secondary side anyway.

Edit

It would be great to know that I can touch the secondary side without being hurt.

The key is to maintain proper isolation between the primary and secondary. Your transformer will do this. But also be sure to maintain a proper "creepage" distance between any wires on the primary side to any wires on the secondary side. Unfortunately I can't tell you the required separation off the top of my head.

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  • \$\begingroup\$ Thank you very much for your answer. Just to be sure - - when you say "In this circumstance", do you mean when I touch the primary side of the transformer, or the secondary? It would be great to know that I can touch the secondary side without being hurt. Sorry to be such a wuss. \$\endgroup\$ – David Apr 17 '13 at 18:42
  • \$\begingroup\$ Touching a wire at less than about 50 V is pretty unlikely to hurt you (but don't go doing it just for fun). 15 V is almost surely safe, you probably won't even feel anything if you don't work at it. \$\endgroup\$ – The Photon Apr 17 '13 at 18:56
  • \$\begingroup\$ Thanks. The first resource linked by Relianthe suggests the same. \$\endgroup\$ – David Apr 17 '13 at 19:18
  • \$\begingroup\$ Before touching the secondary, I would make absolutely sure that the transformer provides safe isolation as per the datasheet. As for clearance between primary and secondary circuits: The best approach is probably to leave as much space as you can. From what I know (warning: not an expert, no guarantees), 8mm of clearance between primary and secondary traces on PCB surface is enough to satisfy the safety standards in almost all cases, but more never hurts. \$\endgroup\$ – Medo42 Oct 22 '14 at 9:44
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I'm just Googling this mostly.

1) That's definitely not a healthy current, to say the least. Depending on the frequency, you can handle different amounts of current.link

2)It's a step down transformer(I think), so this link, says "If the secondary were shorted (the resistor replaced with a wire), the primary current would be much greater, and the secondary current would be 10 times the primary current."

3) I talked to a senior engineer at work, and he said to use a 2A fuse. I think you would put it between your power source and the transformer. That's what it looks like in this design. Scroll down a bit. Take a look at this.

4) I think those are called regulated power supplies.

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  • \$\begingroup\$ Thank you for those great links, especially for the first one. \$\endgroup\$ – David Apr 17 '13 at 19:16
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  1. What does it mean when they say the laptop requires a rated power supply? Does it mean the power supply should employ a stabilizer?

I believe "rated" is UK English for "listed", i.e. it means a listing agency such as CSA, Underwriters Laboratories, or competitor has inspected and approved the power supply design as quality and safe. Obviously this would not be applicable to a homebrew power supply, unless you are very, very serious about your hobby.

There is something to be said for using a listed, off-the-shelf high voltage section for your power supply, so a sealed unit is handling the dirty work of converting from 240V to say 20V AC, and then you take it from there.

Voltages under 30 volts are not likely to harm you as it just doesn't have enough voltage pressure to push a meaningful amperage (10ma) through the human body. That said, humans are not resistors, more like VBOs, as voltage increases, our resistance drops and current rises. That is why 120/240V mains voltage is deadly, but it's rather hard to get electrocuted by a car battery.

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